Classic Chocolate Mousse
4 ounces semisweet chocolate, either chips or cut into small pieces
2 tablespoons unsalted butter, cut into small pieces
2 tablespoons brewed coffee or espresso (optional)
2 large eggs, separated
1/8 teaspoon cream of tartar
4 tablespoons sugar, divided
½ teaspoon pure vanilla extract
½ cup heavy whipping cream
Optional flavor add-ins:
2 teaspoons flavored liqueur (orange, coffee, Irish cream, hazelnut, etc.)
Place the chocolate, butter and coffee in a medium-sized stainless steel bowl and set it over a saucepan of simmering water, so that the bowl completely covers the pan. Melt the chocolate while stirring in the butter and coffee, and any additional flavoring, until the chocolate is completely melted. Remove from heat and allow to cool for a few minutes at room temperature.
Whisk in the two egg yolks until well combined and the chocolate is glossy in appearance. Cover bowl and refrigerate.
Meanwhile, use a handheld or stand mixer (with the whisk attachment) to whip the egg whites with the cream of tartar until foamy. Gradually add 2 tablespoons of sugar and continue to beat on medium speed until the egg whites form stiff (but slightly wilted) peaks and appear glossy and not dry. Set aside.
In another bowl, use a handheld or stand mixer (with the whisk attachment) to whip the heavy cream, remaining 2 tablespoons of sugar and vanilla extract until soft peaks form.
Remove the chocolate mixture from the refrigerator and gently stir a couple of spoonfuls of the beaten egg whites into the chocolate mixture to lighten it. Use a rubber spatula to gently fold in the remaining whites into the mixture until thoroughly incorporated.
Fold in the whipped cream and serve immediately or, for best results, refrigerate for at least 2 hours or overnight. Serve in small bowls or glasses (martini, flutes, champagne goblets) topped with additional whipped cream, fresh berries and/or shaved chocolate. Best served within 24 hours of making.